1. Field of the Invention
This application pertains generally to radiation curable and photocurable compositions. More specifically, this application pertains to compositions which are curable with ultraviolet light, using photoinitiators which form cationic catalysts.
2. Description of the Prior Art
It is well known that coatings play a useful role in the manufacture of a wide variety of useful articles. Until recently, nearly all coatings were formulated and applied by employment of an organic solvent, which often comprised a major portion of the total formulated coating. After the coating is applied to the article to be coated, the organic solvent is evaporated, leaving the dried coating on the article to serve its decorative or functional purpose. This coating system has met with increasing disfavor as the cost of energy needed to evaporate the solvent at the rate required by industry increased, as the price of the solvent increased, and as the deleterious environmental effects of the evaporated solvent became better understood. In addition, governmental regulations have placed ever increasing restrictions on the amounts and types of solvents or organic volatiles permitted to escape into the atmosphere from coatings' compositions. Systems aimed at solvent recovery to reduce pollution and conserve solvent have generally proven to be energy intensive and expensive.
Considerable efforts have been expended by those skilled in the art to develop coating compositions having a minimal amount of volatile organic components and this has led to development of powder coatings, radiation-curable coatings, water-borne coatings and high solids coatings. In these recent developments, the amounts of organic solvents present are minimal and consequently there is little or no atmospheric pollution.
Among the new coating systems, radiation-curable coatings, usually cured with ultraviolet light or electron beam radiation, offer a variety of advantages. They require only minimal energy to effect cure (change from liquid to solid state), they do not contain volatile solvents, and thus do not cause deleterious effects to the environment, and they are cost effective, since essentially all of the applied liquid is converted to a solid coating.
An important disadvantage of photocurable systems is the frequent requirement that the curing process be conducted in an inert atmosphere because of the inhibiting effect of oxygen. Also, most photocurable systems based on acrylates are irritating to the skin and eyes of workers using them and can cause sensitization of those who are exposed to the systems.
Responding to such problems, those skilled in the art have devised photocurable coatings which cure through a mechanism termed cationic polymerization. In these systems, the starting materials are mixed with catalysts which form acids when exposed to ultraviolet light; the starting materials are therefore polymerized via cationic catalysis.
Epoxy resins, linear vinyl ethers, and cyclic vinyl ethers have been shown to be suitable starting materials for photocure via cationic polymerization, as disclosed in, for example, U.S. Pat. No. 3,794,576; the publication of Crivello et al., "New Monomers for Cationic UV-Curing", Conference Proceedings, Radiation Curing VI, pages 4-28, Sept. 20-23, 1982 (Association for Finishing Processes of SME); and British publication GB 2,073,760A.
Crivello et al. reported that diaryliodonium and triasylsulfonium salts could be used in relatively low concentrations as photoinitiators for UV curable coatings based upon multifunctional linear vinyl ether monomers. The cationic copolymerization of such vinyl ethers with epoxy monomers is also reported.
Although mixtures of cycloaliphatic epoxides and cyclic vinyl ethers are considered to be rapid curing in nature, with curing line speeds of 30 to 60 feet/minute readily attainable when a single light source is used, even further increases in curing speed are desirable to provide more productive, efficient and cost-effective industrial coating processes.
In contrast to the cyclic vinyl ethers and cyclic epoxides, cycloaliphatic ethers have been reported as slow in polymerizing under cationic photocuring conditions. See, e.g., Crivello and Lam, "Photoinitiator Cationic Polymerization by Diarylchloronium and Diarylbromonium Salts," Journal of Polymer Science, Polymer Letters Edition, Vol. 16, pp. 563-571 (1978), in which the polymerization of tetrahydrofuran when exposed to ultraviolet light in the presence of a photoinitiator was reported as very slow, particularly in contrast to the polymerization of the epoxide cyclohexene oxide.
Similarly, the applicants have observed that when a substituted cycloaliphatic ether such as 2-methoxytetrahydropyran alone is combined with an onium salt photoinitiator and exposed to ultraviolet light, only slow polymerization takes place.